During the exploration for oil, gas, and other mineral deposits in the subsurface of the earth, seismic waves are used to map subsurface geologic structures and stratigraphic features. Sources, such as explosive charges, surface vibrators, and other energy sources operated on the surface, are used to create seismic waves in the earth. Most land seismic exploration is conducted by locating an array of sensors, called geophones, on the surface of the earth. The geophones detect waves generated by the sources and reflected back from geological structures in the subsurface. The signals are recorded and processed in various ways to yield information about the subsurface of the earth.
During various types of seismic exploration, such as reverse vertical seismic profiling and cross borehole imaging, sources are placed in a wellbore (borehole). The simplest downhole sources are explosive charges and airguns. However, these sources present several problems when generating seismic waves in a borehole. Although these sources can be used for crosshole imaging, they do not possess sufficient power for reversed vertical seismic profiling except in small scale applications. Another problem encountered when using conventional downhole sources is that they (especially explosives), may damage the cement bond or casing in the borehole. This potential to cause damage to the borehole causes a reduction in the amount of energy that can be supplied by the source. A third problem is that conventional downhole seismic sources, especially the airgun, typically expend a substantial fraction of their energy generating tube waves that travel up and down the borehole. This makes the sources less efficient than desired. This feature of creating mainly tube waves rather than radiant P-wave and S-wave energy is shared by most other downhole sources, including magnetostrictive and piezoelectric sources which may be useful for high resolution cross borehole work.
U.S. Pat. No. 4,671,379 to Kennedy, et al. illustrates a different kind of downhole seismic energy source. Energy is supplied downhole from the surface through coil tubing connected to a rotary valve located on the device downhole. A column of fluid in the wellbore is excited by the supplied energy to produce a resonant standing wave. This is accomplished by isolating the fluid between two gas bladders to form a column of fluid and exciting the fluid in the column into an oscillating motion with a driver that is in communication with the column of fluid. The fluid is oscillated at the resonant frequency of the column defined by the two gas bladders. During the operation of the invention, it is desirable to sweep the rotary valve through a range of resonant frequencies in order to gain more information about the subsurface of the earth. In order for this invention to perform a frequency sweep and maintain the resonant frequency of the fluid in the column, the length of the column must change for the various frequencies in the sweep. The device accomplishes this task by physically moving the gas bladders during the sweep. In order to maintain the column at 1/2 wave resonance, the gas bladders are moved during an approximately 45 second sweep through a total distance of about 100 feet. The device must maintain resonance in order to operate effectively.
The source is intrinsically powerful (since its effective radiating length is tens of feet) and does not cause borehole damage. The patent states that the device provides a relatively efficient source of energy by operating at the resonant frequency of the column of fluid. However, several problems arise from this approach. First, mechanical movement downhole is necessary because the source must operate at the resonant frequency of the column and cannot vary the resonant frequency without changing the column length. The device must contain relatively complicated downhole equipment in order to vary the length of the column. A system requiring downhole moving parts such as this one is less reliable than desired. Repairs can only be effected by removing the system from the borehole leading to costly down time in operations. A second problem is with the duration of the sweep time of the device. As a sweep is made the length of the column varies. The distance each bladder must travel during a sweep is approximately 50 feet (based on a half wave length of a resonant standing pressure wave). Therefore, the sweep requires an extended time, approximately 45 seconds. It is not possible with this system to produce short sweeps of a few seconds each. Nor is it possible to operate with an impulsive source as the exciter.
The Kennedy et al. patent also describes an alternate embodiment that does not vary the length of the column. In this embodiment, inflatable sleeves surround the conduit between the end elements. These sleeves may be inflated with air causing a change in the apparent compressibility in the borehole fluid. The change in fluid properties changes the resonant frequency of the cavity. However, in this case also, the system can only execute relatively long sweeps and requires downhole moving parts leading to lower field reliability.
Therefore, there still remains the problem of providing a more powerful downhole source without the complication of moving parts and with increased reliability and flexibility of operation.